The present invention relates to a multi-layered piezoelectric device. More particularly, the present invention relates to a multi-layered piezoelectric device which has a very high piezoelectric characteristic, which is superior in transmitting oscillation between the ceramic substrate and the piezoelectric portion, and which provides a small and highly integrated actuator or sensor, as well as to a process for the production thereof.
In recent years, piezoelectric devices have been used in ink jet printer heads, speakers, microphones, etc. Known piezoelectric devices generally have a piezoelectric portion made of a piezoelectric ceramic composition formed on a ceramic substrate and electrodes electrically connected to the piezoelectric portion. Recently, multi-layered piezoelectric devices have come into use, including piezoelectric portions and electrodes alternately formed on a substrate in a large number of layers, which produce a high output at a low voltage.
Meanwhile, various investigations have been made with respect to the piezoelectric ceramic composition constituting the piezoelectric portion. For example, Pb(Mg1/3Nb2/3)O3—PbTiO3—PbZrO3 ternary solid solution system compositions, or piezoelectric ceramic compositions wherein in the above compositions, part of Pb is substituted with Sr, La or the like, are disclosed in JP-B-44-17103 and JP-B-45-8145. It is expected that such a composition can provide a piezoelectric device in which the piezoelectric portion, as the most important element determining the piezoelectric characteristic of piezoelectric device, has per se an excellent piezoelectric characteristic (for example, in piezoelectric d constant).
Conventional piezoelectric devices were produced by forming a piezoelectric material layer made of such a piezoelectric ceramic composition on a ceramic substrate and then heat-treating the resulting laminate. As a result, the piezoelectric portion has a low density due to the restraint of the substrate, and there have been problems in that the flexural displacement is low, or when a voltage has been applied, dielectric breakdown occurs at a low density moiety. This problem is particularly prevalent in multi-layered piezoelectric devices, and an improvement therefor has been strongly needed.
Under such a situation, JP-A-11-29357 discloses a conventional piezoelectric device obtained by heat-treating a piezoelectric material made of the above-mentioned piezoelectric ceramic composition to form a piezoelectric portion, and attaching the piezoelectric portion to a ceramic substrate to attain densification of piezoelectric portion.
In this piezoelectric device, however, an inorganic or organic adhesive must be used in attaching the piezoelectric portion to the substrate. As a result, there have been cases where the adhesive hinders the transmission of oscillation between the substrate and the piezoelectric portion, or the adhesive component deteriorates the characteristics of the piezoelectric portion and the substrate. Further, in conventional multi-layered piezoelectric devices, no consideration has been made with respect to using piezoelectric ceramic compositions of different formulations in individual piezoelectric portion layers, and sufficient piezoelectric characteristics have not been obtained.